The construction of an inboard motor boat normally requires the propeller shaft to pass through the rear portion of the bottom of the hull. A non-jointed or straight propeller shaft passes through the hull with a packing gland and connects the transmission of the engine to the propeller. In order to reduce the amount of stress on the packing gland and the opening of the hull where the propeller shaft passes through, the propeller shaft must be supported somewhere between the engine transmission or the propeller shaft packing gland (if the packing gland is of the rigid type) and the propeller. The assembly for supporting propeller shafts is commonly referred to as a strut support.
The strut support for propeller shafts taught by the prior art consists of three basic elements: the palm, the strut section and the propeller shaft bearing. The palm is typically a flat mounting plate that is attached to the bottom of the hull. The strut section is another flat plate attached perpendicularly to the center line of the palm and connecting the palm to the propeller shaft bearing housing. The propeller shaft bearing is normally a hollow metal tube, the housing, containing suitable bearing material that supports the propeller shaft.
The above design, while fairly simple in construction and inexpensive, presents many disadvantages. First, the only means for support against shocks and stress in the transverse direction (i.e. perpendicular to the propeller shaft in the horizontal plane) on the strut section is the connection between the strut section and the palm. As noted above, the upper end or edge of the strut section is welded or cast perpendicularly to the palm. Thus, forces in the transverse direction, through leverage, can render significant stress on the strut/palm joint. If the strut is merely welded or cast to the palm, the weld or casting can fail resulting in damage to the propeller shaft, hull bottom and engine.
Conventional strut supports are vulnerable to forces in the longitudinal, transverse, (i.e. from the stern to the bow) and vertical directions as well. In effect, the integrity of the conventional strut support rests on the single connection between the strut section, the palm, and the palm bolted or screwed to the bottom of the boat. Even if the three components (i.e. the palm, the strut section and the propeller shaft bearing) are assembled as a single casting, the palm/strut section can, and does, fail from external forces.
Further, it is often desirable to present the propeller behind the transom to reduce propeller generated noise. When the propeller is located beneath the hull, shock waves cause noise or vibration against the hull. As noted above, conventional strut supports require the palm of the strut support to be mounted to the bottom of the hull forward of the transom. Therefore, in order to present the propeller behind the transom, a significant length of propeller shaft between the strut support and the propeller must be exposed in an unsupported condition thereby leaving the exposed portion of the propeller shaft vulnerable to damage.
Thus, the conventional strut supports are deficient in at least two respects. First, the strut section being attached perpendicularly to the palm leaves the entire strut support assembly vulnerable to forces in the transverse longitudinal and vertical directions relative to the strut section. Second, the conventional strut supports require the palm, and the entire support for that matter, to be mounted to the underside of the hull. If it is desired to present the propeller behind the transom, conventional strut supports leave a substantial length of propeller shaft exposed and unsupported and therefore vulnerable to damage which can result in expensive repairs.